As a mill architect Philip Sidney was soon extremely successful, and was, without doubt, the greatest in the Oldham area. He benefited by the innovations made both by his father, and by Edward Potts (1839-1909), with whom he shares the title of Lancashire's greatest mill architect.

In total he designed about 28 mills in the area, with some 2.7 million spindles, this figure accounting for 40% of the new spindles laid down between 1887 and 1914.

In 1884 he designed the extension to the Stockfield Mill, and in the following year built the Falcon on Victoria Street. This was to be Chadderton's only weaving shed. The Rose Mill at Coalshaw Green was also erected in 1885, and four years later the Richmond Mill was built nearby, at Stanley Road.

In 1898 the Nile Mill, on Fields New Road, was to become a notable feature of the local landscape. This impressive mill, which still stands, had several claims to fame. At its opening it was the largest ring spinning mill in the world, and was also the last mill to be built with the traditional beam engine, gearing and vertical drive shaft. The engine, at 2,500 hp, was also the most powerful beam engine to be installed in an English cotton mill.

In 1906, his continuing success in business enabled Philip Sidney to purchase Stanton Court, near the picturesque village of Broadway, Gloucestershire, an estate on which he settled in 1913. At various times he held several directorships within the cotton industry, and he was also to be honoured as a Fellow of the Society of Architects. In the public sphere he also served as a Justice of the Peace.

The 20th century was to see Philip Sidney design three further mills in Chadderton. The Raven, off Eaves Lane, was constructed in 1907, whilst in the following year the Gorse Mill was built at Whitegate Lane. Its neighbour the Ace Mill was built in 1914, but it was not brought into use as a cotton mill until after the end of the First World War. During the war years it was used for aircraft assembly, as part of a comprehensive, and somewhat secret, government scheme in that area of Chadderton. The complete restoration of the Busk Mill, in 1912, is another of Stott's success stories.

In the Edwardian period alone, Philip Sidney was responsible for 18 of the 66 new mills erected in the Oldham area between 1900 and 1915, although a rival architect, Frederick Whittaker Dixon (1854-1935), exceeded this with his 23 mills. In addition Philip Sidney was responsible for a further 55 mills in other parts of Lancashire, the sum total of all his mills accounting for 44% of the increment in the spinning capacity of the county between 1887 and 1925.

In total he designed more than 124 mills, both spinning and weaving. Twenty-eight of these commissions were abroad, and this overseas work included places not only in Europe, but in the Far East and South America. All his new work was in addition to the many extensions to existing mills, with which he was involved throughout his career. Philip Sidney Stott's most notable trade mark was the use of two corbelled rings on the chimneys of his Mills, instead of the usual one, but he also favoured water towers in the Hotel-de-Ville style. None of his unique chimney features survives in Chadderton, but the Gorse Mill and the Stockfield Milt both show evidence of his architectural style in tower construction. He was one of the last mill architects to adopt the concrete floor, and relied instead on the triple brick arches, supported on steel beams.

In his later life, as a country gentleman in Gloucestershire, he played a major role in the activities of the Cirencester and Tewksbury Conservative Association, and was, for a time, president of the local Association. In 1920 he was knighted, becoming a baronet, and took the title Sir Philip Sidney Stott, thus bringing his first Christian name into official use. He was the only member of the family to be honoured by becoming a member of the Royal Institute of British Architects, whilst his involvement in public life reached its pinnacle when he served the office of High Sheriff of Gloucestershire in 1925-26.

Sir Philip Sidney's death occurred on 31st March 1937, at the age of 79, in a nursing home in Cheltenham. He had been there for a week, during which time he had undergone a slight operation, but had succumbed to the complications which had set in.